The United States will leave around 200 troops in Syria for a period of time, the White House has announced, as President Donald Trump backed away from a complete withdrawal of forces from the war-torn country.

"A small peacekeeping group of about 200 will remain in Syria for a period of time," White House spokeswoman Sarah Sanders said in a statement on Thursday.

The announcement came amid fierce criticism of Trump's decision in December to withdraw the 2,000 US soldiers fighting the Islamic State of Iraq and Levant group (ISIL, also known as ISIS) in Syria, with members of his own Republican Party blasting the move. 

Critics have decried a number of possible outcomes from a precipitous withdrawal, including a Turkish attack on Kurdish-led Syrian Democratic Forces, Washington's main ally in the fight against ISIL, as well as a resurgence of the armed group.

The decision to keep US troops in Syria was announced after Trump spoke by phone to Turkish President Recep Tayyip Erdogan.

A White House statement said the two leaders agreed, regarding Syria, to "continue coordinating on the creation of a potential safe zone".

Turkey wants to set up a safe zone with logistical support from allies and says it should be cleared of the US-backed Kurdish YPG militia, which Ankara considers a "terrorist" group tied to the outlawed Kurdistan Workers' Party (PKK) inside its own borders.

David Des Roches, associate professor at the Near East South Asia Center for Security Studies, told Al Jazeera the US decision was a "political" gesture.

"It's not a large number of forces. It's too small to be militarily significant. So it has to be political," the former Pentagon official said from the city of Bethesda, near the US capital, Washington.

The US troops' task will be to keep the peace between Turkey and Kurdish forces, he said.

"There's two possible messages here. One is to the Turks, yes, we are serious about protecting your interests and we are willing to keep our forces here to restrain the Kurdish elements of the Syrian Democratic Forces, from allying themselves with the PKK and attacking Turkey.

"And the second message is to the YPG that you are not going to be abandoned, we will have a presence on the ground to restrain the Turks from attacking you. So in practice, it looks like these guys will be there as hostages to both sides to keep them on their best behaviour."

A senior US administration official said Trump's decision had been in the works for some time. It was unclear how long the 200 troops would be expected to remain in the area or where exactly they would be deployed.

Leaving even a small group of US troops in Syria could pave the way for European allies to commit hundreds of troops to help set up and observe a potential safe zone in northeast Syria.

"This is a clear direction to our allies and coalition members that we will be on the ground in some capacity," the senior administration official told Reuters news agency.

On Thursday, acting Pentagon chief Patrick Shanahan met his counterpart from Belgium. Before the meeting, Didier Reynders, Belgium's minister of defence, was asked whether he would be open to keeping troops if there were no American forces left.

"We are waiting for preparation of the withdrawal of US troops and we are waiting now for more discussions," he said.

Until now, European allies have baulked at providing troops unless they received a firm commitment that Washington was still committed to the region.